It used to be that the tank taunts either zel or ary and the other is pulled to the wall by their shared chain. (I know I'm not misremembering because I checked on some older youtube videos of the fight)
This is no longer happening, both dogs stay with the tank.
I'm less sure of this next part is unintentional but it seems like it might be - the dogs come off their chain earlier than they did in those old youtube videos, at around 75% boss health compared to 25%. Also, sometimes they both just stay where they spawned without going anywhere at all. Helps the group a lot actually when that happens!